Fixes indentation of empty lines in block comments.

Block comment indentation of empty lines regressed, as we did not
have a test for it.
 /* Comment with...
  *
  * empty line. */
is now formatted correctly again.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182757 91177308-0d34-0410-b5e6-96231b3b80d8